1. Field
Embodiments of the invention relate to the field of yieldable connecting rods; and more specifically, to automatic release mechanisms for connecting rods.
2. Background
Minimally invasive surgery (MIS) (e.g., endoscopy, laparoscopy, thoracoscopy, cystoscopy, and the like) allows a patient to be operated upon through small incisions by using elongated surgical instruments introduced to an internal surgical site. Generally, a cannula is inserted through the incision to provide an access port for the surgical instruments. The surgical site often comprises a body cavity, such as the patient's abdomen. The body cavity may optionally be distended using a clear fluid such as an insufflation gas. In traditional minimally invasive surgery, the surgeon manipulates the tissues by using hand-actuated end effectors of the elongated surgical instruments while viewing the surgical site on a video monitor.
The elongated surgical instruments will generally have an end effector in the form of a surgical tool such as a forceps, a scissors, a clamp, a needle grasper, or the like at one end of an elongate tube. The surgical tool is generally coupled to the elongate tube by one or more articulated sections to control the position and/or orientation of the surgical tool. An actuator that provides the actuating forces to control the articulated section is coupled to the other end of the elongate tube. A means of coupling the actuator forces to the articulated section runs through the elongate tube. Two actuators may be provided to control two articulated sections, such as an “arm” that positions the surgical tool and a “wrist” the orients and manipulates the surgical tool, with means for coupling both actuator forces running through the elongate tube.
It may desirable that the elongate tube be somewhat flexible to allow the surgical instrument to adapt to the geometry of the surgical access path. In some cases, the articulated sections provide access to a surgical site that is not directly in line with the surgical access port. It may be desirable to use cables as the means of coupling the actuator forces to the articulated sections because of the flexibility they provide and because of the ability of a cable to transmit a significant force, a substantial distance, through a small cross-section. However, a cable is only able to safely transmit a limited force. Thus it is generally necessary to provide a means for limiting the amount of force applied to the cable.
In a surgical application, the cable may be driven through an input range of motion at an input end by an actuator. The input range of motion is intended to drive an end effector, such as a surgical tool or articulated joint, through a corresponding output range of motion. However, the end effector may be prevented from moving, such as by contacting a solid obstruction. Thus the end effector may hold the output end of the cable in a fixed position, which may be at the end of its range of motion, while the actuator attempts to move the input end of the cable through its full range of motion. This will result in breakage of the cable without a protective mechanism.
Backdrivability, the ability of the mechanical system to move the input axis from the output axis, is one possible protective mechanism. However, a cable driven output lacks backdrivability because forces cannot be reliably transmitted by pushing on a cable. Without backdrivability, elastic components in series to the actuator output may be added as a protective mechanism. It is difficult to have enough elasticity and enough output force simultaneously.
A cable of small diameter, such as would be used to transmit motive forces to the end effectors of a laparoscopic surgical instrument, needs to be able to transmit forces that are close to the safe working limit of the cable. Thus, a protective mechanism for the cable must allow forces to be transmitted up to the protective limit and then prevent the forces from increasing significantly thereafter while allowing a full range of input motion.
In view of the above, it would be desirable to provide an improved apparatus and method for limiting forces applied to cables that keeps the cable at or below its load limit with the output end held at an end of its range of motion while the input end moves through its full range of motion.